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One of the factors that makes us humans special are our wonderful hands, with their opposable thumbs. On average, about 90 per cent of us prefer to use our right hands, leaving the left-handers in very much a minority at about 10 per cent of the population.

But what about our faces? Why is one side more attractive than the other? What's going on?

The first thing to realise is that this whole "left-right" thingie is a little fuzzy. Sure, 90 per cent of us are right-handed, but 80 per cent are right-footed, 70 per cent are right-eyed while only 60 per cent are right-eared.

The second thing to realise is that in our skull we have two brains, which are mostly not connected to each other. There are about 50 billion neurons in our left brain and the same in our right brain, but only a very tiny connection of about one quarter of a billion nerves joining the two halves.

One result is that our faces are not symmetrical — in other words, the right side of your face is a little different from the left side of your face. This difference increases as you get older. The famous designer, Coco Chanel, supposedly said that "at 20 you have the face that God gave you, at 50 you have the face that you deserve".

Third, our brains are cross-wired to our bodies. What this means is that our left brain mostly controls the right side of the body, and vice versa.

So now, let's look at the study which tried to see what the story was with the attractiveness of our asymmetrical faces. This study was carried out by Professors Kelsey Blackburn and James Schirillo at Wake Forest University in North Carolina.

They photographed 20 volunteers (aged 35-65) on both their left and right faces — not fully side on at 90 degrees from the centre line, but from about 37 degrees off to one side. To compensate for observers who simply preferred the right or left face, the scientists used Photoshop to mirror-reverse the faces.

And then they got 37 undergraduate psychology students to rate the attractiveness of these photos. The professors did more than just ask the students what they thought. They also examined the pupils to see if they changed in size when rating each face for attractiveness. You see, it's pretty well established that the size of the pupil is related to the emotional intensity that an image evokes in you.

So the students were each shown four sharp images. Two were the untouched mostly right face and an untouched mostly left face. The other two were the mirror-reversed right face (which was actually the left face before it had been Photoshopped) and a mirror-reversed left face (which was actually the right face).

The results were interesting. In general, the student judges preferred the left face of the both men and women — regardless of whether they were shown the untouched left face, or a left face which had been mirror-reversed to look like a right face. This was regardless of the gender of the person in the photo, and the gender of the person doing the judging. And the pupils of the volunteers opened a little larger when they rated the faces more attractive.

This left-face attractiveness is seen (to some degree) in portraits. For example, Dr I C McManus examined nearly 1500 portraits painted in Western Europe between the 16th and 20th centuries, and found that about two-thirds showed the left face, while just one-third showed the right face. Thanks to the wiring of your brain it's your left face that best shows your emotional state.

More recently, a study has shown that between the ages of 10 and 20 years, girls spend twice as much time looking at themselves in the mirror than boys do. I expect that it's at this time they learn to put their best face forward.

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